1. Field of the Invention
This invention relates to integrated circuits, and in particular to a method of fabricating a compact, high performance silicon-controlled rectifier structure.
2. Description of the Prior Art
A silicon-controlled rectifier ("SCR") is also known as a reverse-blocking triode thyristor. SCRs or thyristors are triggered into conduction in only one direction, from anode to cathode, by a pulse of control current. Once activated, the SCR continues to conduct current whether or not the pulse remains present. Like transistors, SCRs have two terminals for working current and one terminal for control current. Unlike transistors, however, SCRs do not require any further control current once they are turned on. When off, the SCR normally blocks current attempting to pass either way between the anode and cathode of the device. As a result of their unique functionality, SCRs are widely applied in programmable read-only memories (PROMs) to blow fuses, as well as in motor control systems, solid state automobile ignition systems and elsewhere.
A lateral silicon-controlled rectifier structure implemented in integrated circuit form can be viewed as a PNPN structure in which a lateral NPN transistor is merged with a lateral PNP transistor. That is, the PNP base also serves as the NPN collector and the NPN base also serves as the PNP collector. Because of the need to form buried contacts to some of the regions, and because of the use of a P conductivity type wafer and N conductivity type epitaxial layer, the lateral SCR structure will include parasitic vertical transistors. For example, the N-type emitter of the lateral NPN transistor will also function vertically, using those portions of the P-type base beneath the emitter and an underlying N-type buried layer to form a parasitic NPN device. Similarly, a parasitic PNP device also will be formed.
In conventional non-self-aligned SCR structures, the lateral NPN base width of the device will be on the order of 2 to 3 microns. This wide lateral base and associated high Gummel number make the gain of the lateral NPN device very low. Accordingly, it contributes little to active SCR operation, in contrast with the vertical NPN device. Furthermore, the lateral NPN device will not contribute significantly to carrying current in the transport because of the substantial amount of stored charge in and underlying the lateral NPN base region. To turn the SCR on, that stored charge must be supplied or removed. Because device switching speed is proportional to the stored charge difference between the on and off states, the resulting devices operate undesirably slowly.